Evolution of Micro-organisms and Plants

Cretaceous Period, 145.0 - 66.0 MYA

		Angiosperms marked a higher evolutionary stage in plant development. Their seeds are enclosed in cases and they have specially developed sex organs (stamens and a pistil), which are usually surrounded by brightly coloured petals and a green calyx. These flowering plants probably originated at a time before the Cretaceous period. They quickly adopted to the cooler climate during the early Cretaceous, and after that they developed at an astonishing rate. By now gymnosperms were starting to decline. Some of them still persisted, but their numbers constantly diminished. The only gymnosperms remaining in large number were conifers, many species of which have survived down to the
Figure 23 Cretaceous Plants [view large image]	Fig. 24 Flower-ing Plants [view large image]	present. Figure 23 shows a typical Cretaceous scene of a forested area. Flowering plants (Figure 24) made true evolutionary history. Not only did they grow and reproduce faster than ferns, cycads and gymnosperms, but they also recovered more quickly after being trampled or munched on by animals.

Their brilliant flowers and fruits attracted insect pollinators and hungry planteaters alike, ensuring efficient reproduction and seed dispersal. Given their new popularity, flowering plants soon warded off overattentive herbivores by arming and protecting themselves with a variety of thorns, coarse bark, minerals and chemical poisons.

All hardwood trees, including all the deciduous trees of the temperate zone and broadleaved evergreen trees of the tropical zone, are angiosperms, although sometimes the flowers are inconspicuous. All herbaceous (nonwoody) plants common to our everyday experience, such as grasses and most garden plants, are flowering plants. Angiosperms are adapted to every type of habitat, including water. The angiosperms are divided into 2 classes: the monocots (e.g., lily) and the dicots (e.g., buttercup). The monocots are almost always herbaceous, with flower parts in multiple of three, parallel leaf veins, scattered vascular bundles in the stem, and one cotyledon, or seed leaf. The dicots are either woody or herbaceous and have flower part usually in fours and fives, net leaf veins, vascular bundles arranged in a circle within the stem, and two

Fig. 25 Mono/Di-cots
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cotyledons, or seed leaves. Figure 25 compares the characteristics of these two classes. The reproductive cycle of the flowering plant is illustrated in Figure 10-32. The seeds are enclosed by fruit for protection. Many so-called vegetables are actually fruits, e.g., tomatoes, beans, and squash. Nuts, berries, and grains of wheat, rice, and oats are also fruits.